1. Field of the Invention
The present invention relates to an optical element, and more particularly, to an optical element suitable for use in an optical system of a video camera, a still video camera and a copying machine.
2. Description of the Related Art
Conventionally, various picture taking optical systems have been proposed in which reflecting surfaces such as concave mirrors and convex mirrors are utilized.
FIG. 1 schematically illustrates a main part of a so-called mirror optical system including one concave mirror and one convex mirror.
In the optical system of FIG. 1, light flux 134 from an object is reflected from concave mirror 131, travels toward the object while being converged, is reflected from convex mirror 132, and then forms an image on image surface 133.
The mirror optical system is based on the construction of the so-called Cassegrainian reflecting telescope, and is intended to shorten the total length of the optical system by folding the optical path of a telephoto lens system composed of refracting lenses by the use of two opposite reflecting mirrors.
In addition, in an objective lens system constituting a telescope, a number of methods in addition to the Cassegrainian method have been known in which the total length of the optical system is shortened by the use of a plurality of reflecting mirrors, based on the principles described above.
Thus, a method for obtaining a compact optical system has been conventionally known in which the optical path is efficiently folded with use of reflecting mirrors in place of a lens optical system having a long total length.
However, in the mirror optical system of the Cassegrainian reflecting telescope and the like, there is a problem that the light from the object is partially eclipsed by the convex mirror 132.
In order to solve the problem, a mirror optical system has been also proposed in which reflecting mirrors are decentered with respect to an optical axis in order to prevent the other parts of the optical system from shielding the passage region of the light flux 134, that is to say, the principal ray of the light flux is separated from the optical axis 135.
FIG. 2 schematically illustrates a main part of the mirror optical system disclosed in U.S. Pat. No. 3,674,334. In this mirror optical system, the center axis itself of the reflecting mirrors is decentered with respect to an optical axis to separate the principal ray of the light flux from the optical axis, thereby solving the problem of eclipse.
The mirror optical system of FIG. 2 includes concave mirror 111, convex mirror 113 and concave mirror 112 in the order of passage of the light flux. These mirrors are originally rotary symmetrical with respect to optical axis 114, as shown by a chain double-dashed line. Only the upper portion of the concave mirror 111 with respect to optical axis 114, only the lower portion of convex mirror 113 with respect to the optical axis, and only the lower portion of concave mirror 112 with respect to optical axis 114 are used to construct an optical system in which a principal ray of light flux from an object is separated from optical axis 114, and the eclipse of light flux 115 is eliminated.
FIG. 3 schematically illustrates a main part of a mirror optical system disclosed in U.S. Pat. No. 5,063,586. In the mirror optical system of FIG. 3, a center axis of the reflecting mirrors itself is also decentered with respect to the optical axis to separate the principal ray of the light flux from the optical axis, thereby solving the problem of eclipse.
Referring to FIG. 3, when a vertical axis of object surface 121 is defined as optical axis 127, center coordinates and center axes (axes connecting centers of reflecting surfaces and centers of curvatures thereof) 122a, 123a, 124a and 125a of the reflecting surfaces of convex mirror 122, concave mirror 123, convex mirror 124 and concave mirror 125 are decentered with respect to optical axis 127. By suitably setting the amount of decentering and the radius of curvature of each surface, the eclipse of light flux 128 from an object due to reflecting mirrors is prevented, so that an object image is efficiently formed on an image surface 126.
These reflecting-type picture taking optical systems contain many parts or components. Thus, in order to obtain a required optical performance, it is necessary that each of the optical parts are accurately assembled. More particularly, according to the picture taking optical system of a type in which reflecting mirrors are decentered with respect to the optical axis for the prevention of an eclipse of the light ray from the object, each of the reflecting mirrors must be disposed with different decentering amounts. As a result, structures for mounting reflecting mirrors thereto become complicated, and extremely precise mounting accuracy is required.
As one of the methods for solving the above problems, a method may be considered in which assembly error of the optical parts is avoided by combining mirror systems into one block.
Hitherto, as examples of such mirror systems, there have been optical prisms such as pentagonal roof prisms and Porro prisms which are used for viewfinder systems, and a color separation prism for separating the light flux from a picture taking lens is separated into three colors of red, green and blue to form object images based on each color of the light on the surface of each image pick-up device.
In these optical prisms, since a plurality of reflecting surfaces are integrated, the reflecting surfaces are placed accurately in relation to one another, so that positions of the reflecting surfaces need not be adjusted.
In these optical prisms, however, there is a problem in that a harmful ghost light associated with an irregular incident light incident from positions and angles other than those of an effective light ray is frequently generated.
A function of the pentagonal roof prism which is often used in a single-lens reflex camera as a typical example of the optical prisms will now be described with reference to FIG. 4. Referring to FIG. 4, there are provided a picture taking lens 101, a quick-return mirror 102, a focal plane 103, a condenser lens 104, the pentagonal roof prism 105, an eyepiece 106, an observer's pupil 107, an optical axis 108 and an image surface 109.
A light flux from an object (not shown) is reflected upward of a camera from the quick-return mirror 102 after passing through the picture taking lens 101 so as to form an image on the focal plane 103 located at a position equivalent to the image surface 109. A condenser lens 104 for forming an exit pupil of the picture taking lens 101 on the observer's pupil is disposed behind the focal plane 103, and the pentagonal roof prism 105 for making an object image on the focal plane 103 into a correct image is disposed behind the condenser lens 104.
The object light which has entered the incident surface 105a of the pentagonal roof prism 105 is subjected to a reversal of an object image from right to left, and then is emitted to the observer's side as the object light by the reflecting surface 105c.
The object light emitted to the observer's side by the reflecting surface 105c passes through an emergent surface 105d of the pentagonal roof prism 105, reaches the eyepiece 106 so as to be formed into a substantially parallel light by a refracting force of the eyepiece 106, and then reaches the observer's pupil 107 so as to be observed.
In the pentagonal roof prism constructed as described above, a ghost light shown by an arrow in FIG. 4 incident at an angle different from that of an effective light is reflected in the order of the roof surface 105b and reflecting surface 105c, is totally reflected from the incident surface 105a, and then is emitted from the lower portion of the emergent surface 105d to the observation side. Since the ghost light differs from the normal effective light ray in the number of reflections, an image turned upside down appears on the lower portion of an observation screen.
In order to remove the ghost light, a light shielding groove 100 is provided in the emergent surface 105d of the pentagonal roof prism 105.
In addition, the overall surface of the prism 105 is covered by black paint except for the incident surface 105a and the emergent surface 105d, whereby a reflecting film to be evaporated onto the roof surface 105b and the reflecting surface 105c is protected from environmental change, such as a change in temperature and humidity, and further, a light ray from the outside of the prism is shielded.
Furthermore, an optical system has been known in which curvature is given to reflecting surfaces of a prism. FIG. 5 schematically illustrates a main part of an observation optical system disclosed in U.S. Pat. No. 4,775,217. This observation optical system is intended to observe scenery of the outer world, and to observe an image displayed on a information display means by superimposing the image on the scenery.
According to this observation optical system, a display light flux 145 emitted from an image displayed on an information display means 141 is reflected from a surface 142 toward the outer world, and enters a half mirror surface 143 having a convex surface. After being reflected from the surface of the half mirror surface 143, the display light flux 145 is made into a substantially parallel light flux by the concave half mirror surface 143, and forms an enlarged virtual image of the displayed image after being refracted and passing through the surface 142, and then enters an observer's pupil 144 so as to allow the observer to recognize the displayed image.
On the other hand, a light flux 146 from the outer world enters a surface 147 which is substantially parallel to the reflecting surface 142, and is refracted therefrom so as to reach the concave half mirror surface 143. A semi-transmission film is evaporated onto the concave half mirror surface 143, and a part of the light flux 146 passes through the concave half mirror surface 143 and enters the observer's pupil 144 after being refracted and passing through the surface 142, whereby the observer visually recognizes the displayed image by superimposing it on the scenery of the outer world.
FIG. 6 schematically illustrates a main part of an observation optical system disclosed in Japanese Unexamined Patent Publication No. 2-297516. This observation optical system is also intended to observe scenery of the outer world, and to observe an image displayed on a information display means by superimposing the image on the scenery.
According to this observation optical system, a display light flux 154 emitted from an information display means 150 passes through a plane 157 constituting a prism Pa and enters a parabolic reflecting surface 151. The display light flux 154 is reflected from the reflecting surface 151 to become a convergent light flux, and forms an image on a focal plane 156. At this time, the display light flux 154, reflected from the reflecting surface 151, has reached the focal plane 156 while being totally reflected between two parallel planes 157 and 158 which constitute the prism Pa, whereby a totally slim optical system can be obtained.
The display light flux 154 emitted as a divergent light from the focal plane 156 enters a half mirror 152 having a parabolic surface after being totally reflected between the planes 157 and 158, and is reflected from the surface of the half mirror 152. At the same time, the light flux 154 forms an enlarged virtual image of the displayed image by a reflecting force of the half mirror 152 and becomes a substantially parallel light flux, and then passes through the surface 157 to enter the observer's pupil 153, thereby allowing the observer to recognize the displayed image.
On the other hand, a light flux 155 from the outer world passes through a surface 158b constituting a prism Pb, the half mirror 152 and the surface 157, and then enters the observer's pupil 153. The observer virtually recognizes the displayed image by superimposing it on the scenery of the outer world.
The principal function of the conventional optical prism such as the pentagonal roof prism is to reverse an image by changing the direction in which a light ray travels. Therefore, the reflecting surfaces of the optical prism are commonly formed by planes alone, and the optical prism does not impart curvatures to the reflecting surfaces, and positively correct aberration on the reflecting surfaces.
In the observation optical systems disclosed in U.S. Pat. No. 4,775,217 and Japanese Unexamined Patent Publication No. 2-297516, a semi-transmission film is used in order to observe a displayed image and recognize an object image, which reduces a transmission light amount of the displayed image. Therefore, as described above, a method is commonly adopted in which total reflecting surfaces are used in order to minimize the loss of the light amounts on each of the reflecting surfaces.
The total reflecting surfaces are often formed by planes alone to simplify construction. From the viewpoint of correcting aberration, however, it is desirable that the reflecting surfaces are also formed into curved surfaces to optimally correct aberration.
However, when total reflecting conditions are to be satisfied with respect to all light rays entering the reflecting surfaces, there is no degree of freedom of the shapes of the reflecting surfaces, so that the aberration is not efficiently corrected on the reflecting surfaces.